1. Field of the Invention
The invention lies in the field of conveying technology and relates to devices and to methods for the transfer of flexible, two-dimensional objects, in particular printed products, between two conveyors.
2. Description of Related Art
In conveyor technology, and in particular in the field of printing technology, it is often necessary to transfer objects from grippers of a first gripper conveyor into grippers of a second gripper conveyor. For example, printed products coming from a printing machine are often held and conveyed in groups, e.g. as a pair of two products, by way of a common gripper, for increasing the conveying rate. For the further processing, they must be singularised. At least individual ones of the products are therefore preferably transferred to grippers of the second gripper conveyor in such a way, that each product is finally held individually.
The objects are often held by a common gripper in a congruent manner (with edges which are aligned onto one another). It may however be advantageous for singularising the commonly held objects, to clamp the objects in the gripper such that their held edges are offset to one another (differentiated conveying).
Different cases of how a gripper-gripper transfer may take place, with regard to the method, are outlined in EP-B 1321410, for example: 1. transfer of two products held by a common first gripper to two consecutive second grippers; 2. transfer of each second product pair to two consecutive second grippers and subsequent division also of the product pairs remaining in the first gripper; 3. transfer of only one of the products of the product pair in each case to a second gripper and further conveying the non-transferred product by the first gripper.
Separating objects which are conveyed in pairs, but also the transfer of individual objects from one first gripper to a second one, requires particular demands to the extent that the object edge which is not gripped, is positioned in a reliable manner such that it is led in an as accurate as possible manner into the open gripper jaw of the second gripper. This is important with flexible products such as e.g. printed products, since on conveying they are only held at one product edge and may change their position due to external influences such as e.g. drafts.
For stabilising the objects, the device according to EP-B 1321410 envisages a flat underlay which is not described in more detail and on which the free object edges drag, whilst the objects are conveyed by the first grippers, and which ends in the region, in which the second grippers approach the free object edges from below.
EP-A 1834911 describes a further transfer device, with which individual objects are transferred by first grippers to synchronously moved second grippers. The second grippers are arranged below the first grippers. The first grippers are opened in a transfer region, so that the objects slide into the co-moved, open second grippers on account of gravity, said second grippers being closed upon this. A stabilisation device, which comprises two spiral or helical rotors orientated in opposite directions, is provided for stabilising the objects during the transfer or in the transfer region. These rotors in each case immerse laterally into the intermediate spaces between the objects held by the first grippers and thus form an essentially vertically orientated two-dimensional support which co-moves with the objects over the whole transfer region, holds these in a straight manner and only ends when the second gripper is closed. The disadvantage of this arrangement is the relatively large volume which is taken up by the lateral rotors, as well as the complexity of the complete stabilisation device. This is because the rotors have a certain extension in the conveying direction as well as transversely to the conveying direction and this extension corresponds roughly to the length of the transfer region or to the rotor diameter. The rotor diameter may not be made infinitely small, since a reliable surfaced guidance of the objects would otherwise not be possible. Finally, also it is only the position of the objects in the conveyor direction which is set by the stabilisation device, whilst the position of the free edge in the vertical direction remains undefined.
If the first gripper conveys more than one object, but not all are however to be released, the objects which remain in the first gripper have to be held in a temporary manner when the gripper opens. For this, EP-B 1321410 suggests two alternative solutions. On the one hand the use of a special gripper is suggested, which comprises two clamping regions which are independent of one another and thus may selectively release an object, whilst the other continues to be held. On the other hand, an additional auxiliary gripper conveyor is suggested, whose auxiliary grippers in the transfer region are moved synchronously with the first grippers and the objects which are not to be released are firmly held during the opening of the first gripper and by way of this are held back in the opened first gripper.
A problem with the differentiated conveying of several objects in a common gripper may be that the object remaining in the gripper does not have the optimal position relative to the gripper jaws or to the pivot axis on account of the initial offset.